The invention relates to a method of determining optimum operating conditions in an electrochemical detector and an electrochemical detector using the method. Electrochemical detectors are used for detecting electroactive substances, i.e., substances which are either oxidizable or reducible. Such detectors belong to the most sensitive and most specific detectors presently available and are of particular advantage in liquid chromatography where they are used to detect the liquid eluting from the separation column.
An electrochemical detector is known, for example, from EP-A-140286. This known electrochemical detector comprises an electrochemical cell into which the liquid to be analysed is introduced and in which three electrodes are arranged: a working electrode, a counter electrode (also denoted as auxiliary electrode), and a reference electrode. The electrochemical process is made to occur at the working electrode and the reference electrode compensates for any change in the conductivity of the mobile phase transporting the substances to be analysed. The potential near the counter electrode is held at a fixed value by a control circuit commonly denoted as "potentiostat". The potential near the counter electrode is sensed by the reference electrode which is connected to the potentiostat. When a substance to be analysed arrives at the surface of the working electrode, a current is developed which is converted by an electrometer to a voltage output which can then be processed by further circuitry.
The current developed at the working electrode for a specific electroactive substance in the electrochemical cell varies with the potential applied across the cell. Generally, the current tends to increase when the voltage is increased. Unfortunately, however, unwanted back-ground currents also increase with the voltage. The sensitivity of the electrochemical detector would be the higher the higher the current measured at the working electrode is, but the increased background at higher voltages has a negative effect on the detector sensitivity. It is thus necessary to find a value of the potential which results in optimum detector performance. In addition to the desire for high sensitivity, it is also desirable that the operating potential is such that slight variations of this potential do not cause substantial changes in the current at the working electrode.
A known way to find optimum operating conditions is to record a "cyclovoltamogram" which is a plot of current at the working electrode versus potential. The cyclovoltamogram is produced by sweeping the potential from an initial value to a final value and sweeping it back again to the initial value with simultaneous recording of the current. The time required for recording the cyclovoltamogram typically is less than a minute. The resulting curve has a comparatively complicated shape; in particular, the branch of the curve corresponding to the forward sweep does not coincide with branch corresponding to the backward sweep. From the behavior of the cyclovoltamogram, an experienced operator can obtain an indication at which potential he can expect good detection conditions. The method has the advantage that it can be performed within a short time interval, but the interpretation of the cyclovoltamogram requires a lot of experience and can therefore only be used by well-trained operators. Another shortcoming of this method is its not always satisfactory sensitivity, caused by the comparatively high signal band width which it requires. Another constraint arising when the electrochemical detector is used in connection with a liquid chromatograph, is the difficulty to interpret chromatograms while doing the potential sweep.